Photocell amplifier



Aug. 16, 1932. v. K. zwoRYKlN E 'rAL 1,872,381

` PHoTocELL AMPLIFIER- olfiginal Filed Sept. 15,` 1928 /Vdulafar and 05d/Zafon Patented Aug. 16, 1.932

UNITED STATE-s PATENT] ort-ICE VIIADIHIB. K. ZW OBY'KIN, OF COLLIN GSWOODQ NEW JERSEY, AND HARLEY A. IAMB, 0F

SAN DIEGO, CALIFORNIA, ASSIGNORS TO WESTINGHOUSE ELECTRIC AND MANU- FACT'URING COMPANY, A CORPORATION OF PENNSYLVANIA PHOTOCELL AMPLIFIER Application led September 13, 1928, Serial No. 805,682. Renewed.- 11113.24, 1931.

Our 'invention relates to picture-transmission systems. More particularly, the invention relates'to the circuits associated with a photo-electric cell for obtaining good rcpresentation of the changes in illumination of the cell by corresponding changes in electric currents.

In any picture-transmission system, the changes in illumination of the photo-electric cell, as the picture is traversed by the pictureexploring device, can never be absolutely abrupt. If the picture contains a perfectly sharp'line separating a dark region from a bright region, the time required for the whole of the area commanded by the exploring device to'travel across this boundary is finite, even when the boundary itself is a sharp line.;

In any system of circuits usually associated with a photo-cell there are one or more conductive bodies connected to the photo-sensitive body in the cell and insulated from the rest of the system except for a high-resistance connection. These connected conductive bodies may beconsidered together as constituting one plate of a condenser. The opposite plate thereof will then include the collector or non-sensitive element in the pho- -to-cell and the conductive bodies connected thereto. The discharge-path of the condenser is over` said high resistance. y

When the photo-cell is' illuminated, the photo-electric surface and the conductors. connected to it become charged by the, departure of electrons under the influence of the light. When the photo-cell becomes dark, the quickest time in which this charge can bedissipated is the time during which the condenser, of which the photo-electric body and connected electrodes constitute one member, can discharge over the connecting resistance. Obviously, therefore, the smaller this resistance, the more rapid the discharge may be, and, therefore, the sharper the line in the reproduction of the picture, corresponding to any sharp line in the picture itself.

This resistance, however, must be fairly large because the potential drop across the resistor is the quantity which must be ampliied to produce the electrical representation of the changes in illumination. For one purpose, a low resistance is desirable; for tl other purpose, a high resistance is desira e;

It is an object of our invention to provide a means for meeting these seemingly irreconcilable requirements.

A picture, including a dark region and a bright region, separated by a sharp boundary, cannot be satisfactorily represented b'y the output of a photo-cell and an amplifier alone,

Abecause the amplier will Inagnlfy only the change in current which occurs during the differently illuminated region. One famil-l iar way of doing this is tointroduce a screen between, the picture and the photo-cell. Every region is then crossed by a plurality of sharp lines of the screen, and the cell is intermittently illuminated when the exploring area travels through such region, even though the region be uniformly bright. A

moving slitted mask between the picture and the cell has also been used for this purpose.

It is an object of our invention to provide a novel means for rendering the input to the amplifier periodic.

It is another object of our invention to employ the means for rendering the input periodic as a means for providing for the requirements of high resistance for some purposes and low resistance for others.

-It is a further object of our invention to' provide an `intermittent shunt for the resistor associated with la photo-cell and to so choose the, frequency of the intermissions that a satisfactory picture will be reproduced.

It is a further object of our invention to provide means for preventing the frequency of said intermissions from impressing'itself control the potential of the condenser, which it has at the beginnin of each discharge, by the illumination of t e 'photo-cell.

Other objects of-our invention and details of the construction will be understood from the following detailed description andthe accompanying drawing, in which;

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Fig. 1 is a diagram of the apparatus and circuits used in practicing our invention.

Fig. 2 is a modification of Fig. 1.

In Figure 1, the rectangular 1, marked Variable Light Source is intended to represent the exploring device by means 'of which-the light from the picture to be transmitted is caused to vary u1 accordance with the brightness of the several points in the picture. Any usual or desired form of eX- ploring device may be used'. The particular exploring device chosen is not a feature of our invention.

Y Practically all such exploring devices receive light'from a small area. in the picture,

and deliver it to the photo-cell. As the e xploring device. travels throughout the plcture, the illumination ofv the photo-cell changes in accordance with the light received from .the various points in the picture.

The varying light is delivered to the photocell 2 which is preferably of the gas-filled type. The photo-electric body 3 in the cell is connected to one terminal of a resistor 4.

This resistor is preferably several megohms. A highpotential battery 5 is connected 1n series between the one end of the resistor 4 and the nonsensitive electrode of the photocell. A small battery 6 is inserted between the battery 5 and the resistor 4. The negative terminal of the battery 6 is connected to the' resistor 4, and thev positive terminal of the battery 5 is connected to the photo-cell. It will, therefore, be apparent that, ifv desired, the battery 6 may be a small portion of the battery 5, but it is ordinarily more convenient `to have separate batteries.

The terminal of the resistor 4, which 1s connected to the element 3, is also connected to the anode 7 of a vacuum tube 8. The cathode of the vacuum-tube may be of the ordinary type, or, if preferred, may be an shown as energized from a source of alternating current represented by the transform-J er 11. p

,shown von the drawing rectangle 27, marked tubes as necessary.

The transformer 11 is also used to supply vheating energy to the heater 12 of the cathode 13 of a second vacuum tube 14. The vacuum tube 8 is provided'with a second grid shown as a screen-grid 15 surrounding the anode 7. A battery 17 is provided which pla-ces a strong positive potential upon the screen grid 15. Preferably, the grid 15 is in the form of two cylindrical sheets of wire mesh, one inside and one outside of the cylin- .drical anode 7, but this specific form is not essential.

The grid 20, which is next to the cathode 9 of the tube 8 serves to control the space current in said tube. A small negative bias is placed upon this rid by a battery 21. Supei posed upon the iasing potential is an alternating potential which is represented as impressed through the transformer 22 from a source 23 of moderately high frequency.

The tube 14 is provided with a grid 25 which is connected to the plate 7, the resistor 4 and to the .photo-electric member 3 of the cell 2. The tube 14 is also provided wit-h an anode 26. Connections from the `anode 26 andthe cathode 13 constitute the output circuit of the tube 14. The ouput circuit includes the usual B battery, not explicitly but included in the Amplifier. The amplier 27 may comprise as .many vacuum he tube 14 is also provided With a screen-grid 28 biased positively by a battery 29, in away closely similar to the screen grid 15 and battery 17 in the tube 8.

The amplifier 27 is connected to the modulator of a system 'represented-by the rectangle 31 and including an usual or desired transmitting device, where y modulated carrier currents are delivered to the antenna 32 and thus transmitted to the receiving'station In the operation of the device, if the photoelectric cell 2 is uniformly illuminated, the metallic body, comprising the photo-sensitive member 3, the anode 7 and the grid 25, will tend to become ositively charged. This tendency will be o set by the current through Y the resistor 4 which tends to bring the potential of said metallic body to a value nearly the same as the negative terminal of the battery 6.

The potential resulting from the two tendencies will persistv as long as the tube 8 is not conductlve. The period during which the tube 8 is non-conducting is determined by the period of the generator 23. The potential of the battery 21 impresses a small negative bias upon the grid- 20 preferably such as to render the tube 8 non-conductive when no potential is impressed upon the grid 20 from' the transformer 22. Even when the anode 7 receives a negative potential from the battery 6, the tube 8 will be slightly conducting because of the eiect of the large be suificient to overcome this.- The potential l from the secondary 22 will produce a nearly proportional change in the conductivity of `the tube 8 when it renders the grid 20 more positive, but when the transformer 22 delivers' a negative potential to the grid 20, the tube 8 becomes absolutely non-conductive.

The frequency of the generator 23 is chosen to correspond to the faithfulness with which the reproduction shall give the details of the original picture. Thus, if the time required for the exploring area to cross a sharp line in the picture is known, the period of the generator 23 should be less than this. We have found that it isA suicient if the period is not greater than one-half o f said time. For the transfer of pictures with a satisfactory degree of detail, it is found that 3,000 cycles per second is sucient frequency for the generator 23.

When the tube 8 becomes conductive, the

positive charge which has accumulated upon 25 the anode 7, grid 25 and photo-sensitive' member 3 tends to discharge through the tube 8. This is in addition to the steady discharge current in the resistor 4. Consequently, the potentials of the anode 7 and the grid 25 3o decrease.

The manner in which the with the time during this dlscharge is well represented by the familiar curve for the discharge of a condenser through a high resistance. T'hat is to say, the decrease of potential is at first very rapid, but afterward it diminishes more slowly.. flhe time during which the potential of such condenser will decrease to Y l ofthe original value, where e is the Naperian base, is called the capacitative time-constant. 4,) In order to avoid being obliged to wait until the discharge is substantially complete, the frequency of the generator 23 is made so high .that the tube 8 becomes non-conducting again lwhile the rate of discharge ofl the capacitative system is still large. Preferably, the time during which the tube 8is` conductive is made smaller than the capacitative time-constant `of the system incuding the capacity of the photo-cell, the grid-cathode capacities of tubes 8 and 14 and the combined impedance of resistor 4and tube 8 in parallel. In this way, the changes in potential on the grid .25 are rapid and the magnitude of them is governed, not bythe constant amplitude 'of the potential from the transformer 22, but by the potential which is' impressed upon the -anode 7 and this is dependent upon the degree of illumination of the photo-cell 2.

C3 While illumination of the photo-cell2 is steady, a stead current from the battery 5 iows through t ephoto-cell and through the resistor 4. Except for the'permanent eiect of the battery 6, which, under these conditions, is small, thepotential impressed across the vacuum tube 8, to operate as a B battery 'for said tube, isthe drop over the resistor 4. The fluctuations in the potential of the .grid 20 produce iuctuations in the potential of the plate 7 which are a function of the B battery potential impressed on this tube, that is, they are dependent upon the steady drop over the resistor 4 just mentioned. y

When the photo-cell 2 is highly illuminated, the cell is conductive. The current through the resistor 4 is, therefore, large. The drop over this resistor is large and the `B battery potential for the tube 8 is large.

Consequently, the amplitude of the potentlal at the plate 7, and therefore, thel amplitude of the potential on the grid 25, is larger than when the Aphoto-cell 2 is but, faintly illuminated.

The output of the vacuum tube 14 is, therefore, an alternating current of the frequency of the generator 23 and .of an amplitude determined by the illumination ofthe photocell 2. The output is not,'however, of the usualV sine-Wave form. It consists of steep parts corresponding to the rapid but partial otential variesA discharge of the condenser, including members 3, 7 and 25, and then a rapid charging thereof. The potential to which itis charged each time is iixe'd by the illumination. The 1'00 potential to which it discharges, being a fraction of the potential when charged, which fraction is determined by the time the tube 8 remains conductive, is also, though to a smaller degree, variable with the illumination of the photo-cell. The picture corresponding to variations in the light source 1 is thus represented in the energy delivered from the antenna 32 by a modulation having a high frequency and an amplitude determined by the brightness of that point in-the picture represented by the transmitting energy at any particular instant. When the photo-cell 2 is completely dark, the potential of the end of the resistor 4, connected' to the photo-cell, will be the potential of the negative terminal of the battery 6, because this batter is then connected to no closed circuit. -T e battery 6 is preferably of a potential to impress upon the grid25 a bias which will bring the tube 14 to the condition corresponding to the lower end of the straight part of the grid-potential-plate-current characteristic. Small iuctuations in the plate current from the tube 14 result from the potential impressed by the secondary 22 upon the grid 20 in the tube 8A while the photo-cell 2 is dark.

The means for synchronizing the sending and receiving stations requires. a special modulation of the output from the antenna 32 at each traverse of the picture. The means for producing this is not shown because it does not constitute a part of the invention covered by this application. This means impresses upon the grid 25 a suiciently negai tive potential to completely cut off the plate i mum change of potential at the grid 25 which,

ybeing amplified, produces maximum modula-` tion of the energy delivered to the antenna 32.

'Even when the periodic changes on the anode 7 are a maximum, they are without effeet across the capacity between the anode 7 and the grid 20, or between the anode 7 and the cathode 9. They, therefore, produce 119 direct eect, through an capacity, upon the grid 25.. The reason or this is that the screen-grid 15 is .so strongly positive that no potential changes on the anode 7 exert an inluence beyond the'screen grid.` The otential changes upon the grid 20v can, there ore, produce a result upon the grid 25 only as they acs)e through the kspace-current effect in the tu 8.

This is ordinarily an amplifying eiect, but y it may, when the potential across the resistor 4 is small enough, be a de-ampliying eiect. By a de-ampliying effect we mean that the change in the plate current is less than the change of potential upon the grid. It has already been Vpointed out that when the photo-cell is dark, the potential between the anode 7 and the cathode 9 is determined by the battery 6 and this may, if desired, be so small that the changes in plate currentare small.

It is, for some purposes, advantageous toseparate the grid 25 from the anode 7 by a condenser 33 as in Fig. 2. When this is done, a resistor 34 and a biasing battery 35 should be inserted toconnect the grid 25 with the cathode 13. Under these circumstances, the biasing battery 6 may be omitted. The drop across the resistor 4 will then be only that due to the very small current which flows by' (lealage through the photo-cell 2 when it is Many variations in the details 'of the construction will readily occur to those skilled in the art. The specic description of but one form of our invention is therefore not to be considered as a limitation. No limitation is intended except such as is required by the prior art or indicated in the following claims.

We claim as our invention:

1. In combination, a photo-cell, a resistor` connected between the electrodes of said photo-cell, a vacuum tube the space-current, path of which is connected in shunt to said resistor, and means for changing the condition of said vacuum tube between conducting and non-conducting at high frequency.

2. In cmbination, a photo-ce1l,'a resistor connected lbetween the electrodes of said photo-cell, a; vacuum tube in shunt to said resistor, means for changing the condition of said vacuum tube between conducting and .non-conducting at high frequency', means for vacuum tube, means for implressing a periodic Y potential upon the grid t ereof and means for impressing between the anode and the cathode thereof a potential controlled by a light-responsive device.

5. In a picture-transmission system, a conductive body forming one memberof a ca pacitative system, a high resistance discharge path for said system and means for providing an auxiliary discharge path of varying conductivity.

6. In a picture-transmission system, a conductive body forming one member of a capacitative system, a high resistance discharge path for said system and means for providmg an Aauxiliary discharge path of varying conductivity, the period during which said auxiliary discharge path is of substantial conductivity being less than the capacitative time-constant of said capacitative system.

7. In combination, a photo-cell, a resistor connected between the electrodes of said cell, a vacuum tube the space-current path of which is connected in shunt to said resistor, means for changing the condition of said vacuum tube between conducting and nonconducting at high frequency, and means for amplifying changes in potential across said resistor.

8. In combination,.a photo-cell, a resistor connected between the electrodes of said cell, a vacuum tube having an input circuit including said resistor, a second vacuum tube the space-current path of which is connected n shunt to said resistor, and means for nemesi Achanging' the condition of said lsecond vacnum tube between conducting and non-conducting/ at high frequency.

9. In combination, a photo-cell, a resistor connected between the electrodes of said cell, a vacuum tube having an input circuit including said resistor, a second vacuum tube ofthe shield-grid type the space-current path of which is connected in shunt ,to said resistor, means for rapidlycchanging the condition of said second vacuum tube between conducting and non-conducting, and means for biasin the shield grid thereof to a potential su ciently positive to prevent changes in the plate potential from appearing as variations in the output current therefrom.

l0. In combination, a photo-cell, a resistor connected between the electrodes of said cell, a vacuum-tube amplifier having an input circuit including said resistor, a potential source in series with said photo-cell and said resistor, a potential source for biasing said amplifier to the cut-ofil point, a vacuum tube the space-current path-of which is connected in shunt to said resistor, and means for changing the condition of the last mentioned vacuum tube between conductive and nonconductive at high frequency. In testimony whereof, we have hereunto subscribed our naines this 14th day of August,

VLADIMIB K, ZWORYIUN. HARLEY A. IAMS.y 

